The present invention relates to optoelectronic transceiver modules and, more particularly, to structures of and methods of making overmolded laminate and other subassemblies in a module for coupling a multiple channel fiber optic cable to a multiple channel Vertical Cavity Surface Emitting Laser (VCSEL) transmitter and to a multiple channel receiver comprising a transimpedance amplifier commonly referred to as a Preamplifier with Integrated Detector (PAID).
An optoelectronic transceiver is the key component in a parallel fiber optic data link. One such transceiver is a modular package or module for coupling a multiple channel fiber optic cable to a multiple channel Vertical Cavity Surface Emitting Laser (VCSEL) and to a multiple channel receiver. The module consists of various components, including both CMOS and optoelectronic dies. It is designed to accept a single connector that has one receive and one transmit section and is mounted on the end of a dual 12-channel fiber optic ribbon cable. The transmit half of the module converts parallel electrical input signals into their corresponding parallel optical output signals through a laser driver and a Vertical Cavity Surface Emitting Laser (VCSEL) diode array. The receive half of the module converts parallel optical input signals into corresponding parallel electrical output signals by using a photo-detector and a transimpedance amplifier to convert the optical input signals to voltage signals. This amplifier is commonly referred to as a xe2x80x9cPAIDxe2x80x9d (Preamplifier with Integrated Detector).
The optoelectronic transceiver module comprises two major building blocks. One is an overmolded laminate subassembly that incorporates electronic functions using standard manufacturing materials. The second is a retainer subassembly that incorporates optical and other components necessary to support the module""s optoelectronic, optical and connector functions. The retainer subassembly includes both a receiver optical subassembly (ROSA) and a transmitter optical subassembly (TOSA). The module is assembled by mounting the retainer subassembly on the overmolded laminate subassembly and electrically connecting the two subassemblies. Finally, a heat sink and an EMI (Electromagnetic Interference) shield are attached.
A number of problems have been encountered in the assembly of the transceiver module and also in the manufacture of the overmolded laminate subassembly, the overmold frame of which is the main structural member of the module. The overmold frame (also referred to as the xe2x80x9covermoldxe2x80x9d) protects the non-optoelectronic dies and their wirebonds, and needs to incorporate mechanical features to locate, align and hold in place other parts of the module.
Since the overmold is a key part of the module with respect to alignment of the other components, it is very important to keep a tight tolerance between the laminate board substrate and the overmold. In other words, components that are aligned with the overmold have interface contacts that must be aligned with corresponding interface contacts on the laminate board. For example, the flexible circuit (flex) connected to an optical die (transmitter or receiver) must be aligned in the cavity of the overmold for its leads to be electrically connected by wirebonds to corresponding pads on the laminate board. Lateral offsets between the flexible circuit leads and the bonding sites on the laminate may result from cumulative placement errors, such as retainer to overmold and overmold to laminate. Excessive offsets between bonding sites can adversely affect wirebond yields and process times, and create a high inductance electrical subsystem due to the longer wires and higher wire loops required to accommodate the lateral offsets.
The present invention provides improvements in positioning a laminate board or other substrate on a mold base to ensure proper alignment of the overmold frame when it is molded onto the substrate. Improved alignment features are also provided in the overmold frame to ensure that positioning of the retainer assembly and the flexible circuit mounted thereon is accurately achieved. These improvements are applicable to transmitter modules, receiver modules, and to combined transmitter and receiver (transceiver) modules.
Prior molding processes use outside edges of the laminate to locate it on the mold base, the tolerance of using the outside laminate edges being xc2x18 mils. The laminate edge is used because it is beyond the area on which the overmold frame is deposited and thereby avoids any interference with the flow of the molding composition.
To decrease the locating tolerance range from about xc2x18 mils to about xc2x12 mils, a pair of concave arcuate recesses, referred to as xe2x80x9cmouse bitesxe2x80x9d, are formed in the leading edge of the laminate, each mouse bite being positioned for abutting a corresponding alignment pin or other convex arc shaped feature on the mold base. The arcs of the mouse bites face in opposite directions and are preferably formed at opposite corners of the leading edge. However, the mouse bites may be located in spaced relation anywhere along the leading edge, and may even be created by opposing arcs within a single recess.
The arc of the mouse bite is drilled, milled or otherwise cut on a radius greater than the radius of the locating pin such that abutment between the pin and the mouse bite is along substantially a line of contact instead of a wide band of contact. Drilling or milling of the mouse bite arc is preferred because drilled or milled recesses provide a tolerance of xc2x12 mils. The overmold frame (also referred to as xe2x80x9cthe overmoldxe2x80x9d) is therefore more accurately positioned relative to the contact interfaces on the laminate board (also referred to as xe2x80x9cthe laminatexe2x80x9d). Thus, other components subsequently assembled on the overmold and aligned to its features will, in turn, be more accurately placed relative to the laminate board features. The mouse bites permit a laminate handling process that is easy and economical, and that reduces yield losses otherwise resulting from too great a tolerance between the overmold frame and the laminate board.
The invention also provides improvements in locating, positioning, and stabilizing the retainer assembly on a platform of the overmold. In conventional designs, a pair of standard holes in the overmold platform is mated with a pair of cylindrical posts projecting from the bottom of the main retainer piece (also referred to as xe2x80x9cthe retainerxe2x80x9d). However, difficulties have been encountered with such standard holes in mating the posts to the holes, in achieving a proper interference fit to frictionally secure the retainer to the overmold, and in inserting the posts into holes containing air and/or liquid adhesive when the overmold platform surface is coated with a layer of epoxy adhesive for permanent attachment of the retainer. In the latter case, the air and/or adhesive is not able to escape from the holes as necessary to allow the two parts to mate correctly.
The present invention provides an elongated hole or slot (slot hole) for receiving one post and a triangular-shaped hole (trilobe hole) for receiving the other post of the retainer assembly. Both the slot and the trilobe holes provide vent passageways in the form of free spaces between the hole walls and the post through which excess adhesive and/or air may escape as the posts enter the holes. The trilobe hole has additional advantages in that it provides three bands of contact between the corresponding post and the hole walls, each of the three sidewalls of the trilobe hole providing a narrow band of contact parallel to the axis of the post.
In addition, the slot hole contacts its post only along two narrow bands of contact, one on each opposing wall. The slot hole therefore provides sufficient freedom of movement of the retainer in the x-y plane for the contact pressure along the three narrow bands of contact in the trilobe hole to be substantially equal. The contact between the posts and the sidewalls of the slot and trilobe holes are preferably along narrow bands instead of along substantially a line because the diameters of the posts are chosen to provide an interference fit, such that a band of contact is created along each sidewall of the holes when a small arc of the post perimeter extending parallel to central axes of the posts compresses the overmold material, which preferably is a molded resin composition. Each of these bands of contact is substantially perpendicular to the plane of the laminate, is preferably about 25 to about 250 microns, more preferably about 100 microns, in transverse width, and have a length of preferably about 0.25 mm to about 0.75 mm, more preferably about 0.4 mm. The retainer posts are preferably cylindrical (round in cross-section), but may have other cross-sectional shapes provided that the shape selected gives equivalent bands of contact with the slot and trilobe holes. The posts have a diameter of preferably about 1.25 to about 2.5 mm, more preferably about 2.0 mm, and a length of preferably about 1.0 to about 2.5 mm, more preferably about 1.75 mm.
The lateral engagements between the retainer posts and the slot hole and the trilobe hole firmly fix the retainer assembly from lateral movement in the x-y plane (eliminating two degrees of freedom) and from rotation around the z-axis (eliminating a third degree of freedom), and the frictional engagements between these holes and the corresponding retainer posts restrain movement of the retainer along the z-axis (eliminating a fourth degree of freedom). The engagements between the retainer posts and the slot hole and trilobe hole also tend to eliminate the two remaining degrees of freedom, namely, rotation of the retainer assembly around the x-axis and the y-axis. However, to positively eliminate these latter two degrees of freedom, a set of three standoff pads projecting from the underside of the retainer are arranged to abut a set of three standoff pads projecting from the platform of the overmold, each set of pads preferably being arranged with one pad at the corner of an imaginary isosceles triangle for maximum stability. The abutting standoff pads also provide a gap of substantially uniform width that is filled with the adhesive, preferably an epoxy resin, for permanently securing the underside of the retainer to the surface of the platform of the overmold.
The present invention also provides a simplification of the TAB bonding process for bonding proximate leads of the flex to the optical dies and their carriers. Conventional integrated flex cable has exposed leads cantilevered off of the flex and these leads must be TAB bonded to the optical dies and their carriers. However, the cantilevered leads may be damaged during shipment significantly complicating the TAB bonding process. One way to protect the leads from such damage is to provide a protective shroud that must be removed prior to assembly and thereby requires an additional process step. The present invention provides a shroud with windows that leave portions of the leads exposed for attachment to each die and carrier, which eliminates the need to remove a protective shroud and thereby avoids this additional process step. This, in turn, improves yields and cycle times and reduces assembly and equipment costs.
Adjacent to the overmold platform for supporting the retainer is an open cavity for receiving the flexible circuit and optical die carrier portions of the receiver and transmitter optic assemblies mounted on and cantilevered from the retainer. The optical coupler portions of these optic assemblies are inserted within and held by the retainer. The overmold of the invention provides features for precisely aligning distal leads of the flex with the receiver and transmitter terminal pad arrays on the laminate board. These alignment features are part of the overmold wall that defines the open cavity providing access to the terminal pad arrays and also to grounding pads for the die carriers of the two optical assemblies. The access provided by the open cavity permits the flex leads to be connected to the terminal pads by wirebonds. The alignment features comprise portions of the cavity wall that are perpendicular to the laminate and are slanted at an angle, preferably about 45xc2x0, relative to a central axis of the flex. Corresponding beveled edges are provided on a stiffener pre-attached to a distal portion of each flex, and the abutment or interfacing between the beveled edges of the stiffener and the slanted portions of the cavity wall precisely locate the distal leads of the flex over corresponding pads of the terminal pad arrays.